The present invention relates to controlled-release products of the slow release type and to processes for the production thereof and, in particular, to encapsulated or coated granular or particulate products which release their nutrient content at a slow, controlled rate over time.
Encapsulated fertilizers are known to be very effective and efficient sources of slow release nutrients for the long-term feeding of plants. The nutrients are released at slow, controlled rates through the fertilizer's coating resulting in a sustained feeding of the plants. As a result, one application of an encapsulated fertilizer can provide the necessary nutrients for a plant that would ordinarily take multiple applications of soluble fertilizers. In addition, encapsulated fertilizers can also be more efficient and cause less environmental concerns than soluble fertilizers due to their slow release of nutrients. Since the nutrients are released at a slow sustained rate rather than a sudden surge, more of the nutrients are absorbed by the plant and thus are not washed away or leached through the soil, where they can enter the ground water.
One type of slow release encapsulated fertilizer currently in wide use for both nursery and turf applications is sulfur coated fertilizer. Coating weights ranging from 15% to 35% by weight are applied to granular fertilizer in coating drums. There are two modes of release of the sulfur coated fertilizer's nutrients. The first is by diffusion through cracks and other imperfections in the sulfur coating. This allows the nutrients to be released rather quickly, and is the predominate mode of release for short-term fertilizer products. The second mode of nutrient release is through coating breakdown. This allows for the longer residual feeding of plants and thus is the primary mode of release for nursery-type sulfur coated fertilizers. The major advantage of the sulfur coated fertilizers is their relatively low cost. This is due to low raw material costs as well as inexpensive manufacturing costs associated with the coating drum process.
A second type of encapsulated slow release fertilizer utilizes solvent applied polymer coatings. For this type of encapsulated fertilizer, the polymer is first dissolved in an organic solvent and then sprayed onto the fertilizer base in either a coating drum or a fluid bed. As the solvent evaporates, a very uniform, continuous polymer film is left behind, forming the fertilizer's barrier coating. Examples of solvent applied polymer coated fertilizers which are currently in use in the nursery industry are disclosed in U.S. Pat. No. 4,019,890, issued to Fugita et al., Apr. 26, 1977 and U.S. Pat. No. 3,223,518 to Hansen on Dec. 14, 1965.
Another type of encapsulated fertilizer that exhibits good slow release properties is latex coated granular fertilizers such as those disclosed in U.S. Pat. No. 4,549,897 issued to Seng et al., on Oct. 29, 1985 and Korean Patent No. 88-153 issued to Mun et al., on Mar. 12, 1988. Such latex coated fertilizers are produced by first applying a sodium silicate precoat to the fertilizer core to protect it from dissolution caused by the water-borne latex. After precoating has been completed, a high molecular weight polymer latex top coat is applied to the precoated fertilizer core. As the water from the latex evaporates, a continuous film, similar to that left behind during solvent-applied polymer coating, is left surrounding the fertilizer granule. It is this latex top coat which gives the encapsulated fertilizer most of its sustained release properties.
Solvent and latex applied polymer encapsulated fertilizers both offer similar important benefits over sulfur coated products concerning consistency of release rates and the ability to provide extended fertilizer residuals. These benefits are chiefly due to the uniform, continuous, and rather defect-free film coating which surrounds the fertilizer core in each product type. These polymer coatings are also very tough and durable and generally are not prone to significant mechanical breakdown. In addition, the coatings are biologically inert and, thus, are not susceptible to breakdown resulting from microbial activity in the soil or other potting media. As a result, the majority of the nutrient release is by diffusion through the polymer coat, rather than release through imperfections and flaws or as a result of particle breakdown. This allows for a much more uniform and consistent nutrient release rate and, if the barrier properties of the polymer are sufficient, a longer residual nursery-type encapsulated fertilizer than sulfur coated products.
However, sulfur coated fertilizers offer certain advantages over soluble fertilizers such as providing somewhat slower release of nutrients, and being less costly than other encapsulated fertilizers. Conversely, there are some disadvantages to the sulfur coated fertilizers, which, as a result of imperfections and coating degradation, tend to be highly variable and lead to inconsistencies in the product's release rate. As the number of coating imperfections increases, so does the fertilizer's nutrient release rate. In addition, the sulfur coating is fairly brittle. With increased handling, the coating breaks down causing further variability and increase in the nutrient release rate.
Another inherent disadvantage of sulfur coated fertilizers is the problem with soil acidulation. As the sulfur coating begins to break down, the free sulfur combines with water to form an acid. This does not create a problem with turf applications where there is plenty of soil available to dilute the effect of the acid, but can be a problem with potted plants where the acid can significantly lower the potting media's pH, which can cause damage to the plants and retard their growth.
Solvent applied polymer encapsulated fertilizers, such as those disclosed in U.S. Pat. Nos. 4,019,890 and 3,223,518 also have problems associated therewith. For example, they require the use of organic solvents to dissolve the polymers before they can be applied to the fertilizer core. This presents an economic disadvantage for the solvent-based polymer encapsulation processes in addition to presenting potential health and environmental hazards.
Organic solvents, such as those used for polymer dissolution, tend to be very expensive. To help offset this high cost, a solvent recovery system must be used in conjunction with the coating process to aid in the reduction of fresh solvent requirements. Although such a solvent recovery system does reduce the amount of fresh solvent required, thus lessening solvent costs, it increases the overall manufacturing conversion cost of the polymer coating process. As a result, the production costs for solvent applied polymer encapsulated fertilizers are higher than processes which do not use solvents thus presenting a problem of economics relative to such processes and their resulting products. In addition, the capital costs for a solvent recovery system are quite high which causes a further economic problem in regard to the commercial application of such encapsulation techniques.
Another problem presented by the implementation of solvent-based polymer encapsulation processes concerns health issues presented by the toxic nature of many organic solvents used therein. These solvents and the vapors they emit can be very harmful to humans. Most organic solvents also are very volatile, posing both fire and explosion hazards. Further, uncontained solvent spills outside the production facility and emissions of vapors into the atmosphere present potential environmental problems. As a result of these potential health and environmental issues, extra caution, as well as increased capital, must be allocated for such processes using organic solvents.
Prior latex encapsulated fertilizers as exemplified by those disclosed in Korean Patent No. 88-153 also have major problems associated therewith. The processing conditions taught therein combined with the physical properties of the water-borne latex top coat employed, do not enable water removal quickly enough to prevent dissolution of the soluble fertilizer core. Therefore, a hydrophobic precoat, such as sodium silicate, must first be applied to the fertilizer core protecting it from dissolution. Once the precoat has been applied, the latex top coat can be applied, but this two-stage coating operation causes a problem of economic feasibility by increasing both the raw material and operating costs which results in a significant increase in product cost.